This invention relates to an improved coated pitch prill and to improved methods and apparatus for producing this prill. Pitch is a well known liquid hydrocarbon residue derived from coal and/or petroleum. A prill is a solid particle or pellet.
Pitch prills are used widely as a binder in the manufacture of electrodes for use in electrochemical processes. Two such applications of prime importance are as binders in preparing carbon anodes for aluminum reduction cells or binders for preparing graphite electrodes for electric steelmaking furnaces. Two serious problems which have plagued this industry are (1) the caking and fusing problem and (2) the dustiness problem of the pitch prills, especially in the handling necessary in storage and shipping. The caking and fusing problem is especially severe when the prills must be shipped or stored in storage vessels such as in the hulls of ships 10 to 20 feet in depth for several days at higher temperatures such as 100.degree. to 130.degree. F. The combination of pressure and temperature over this time period often results in sticking together of the prills (caking) or complete fusion of the prills into a unitary mass which in both cases becomes an extremely difficult, costly and time consuming unloading problem, especially after the pitch has rehardened.
The dusting problem besides being very annoying to workers, creates potential hazards to health due to the noxious substances often found in pitch. Various attempts have been made to solve these problems in the past.
One attempt to overcome the caking and fusing problem is shown in U.S. Pat. No. 2,550,838 wherein the pitch prill is coated with material such as alkaline chlorides and sulfides, finely divided mineral substances, etc. However, the coating materials used in this patent would not be satisfactory for the electrical applications mentioned above due to the impurities added to the electrodes which would result in deterioration of the electrical properties of the electrode and objectional impurities in the product produced by the electrochemical process.
One approach for cutting down on the dustiness problem in handling of the prills is to form a substantially spherical prill by controlled dripping of the liquid pitch into a cooling medium such as water in a manner which would substantially control the spherical shape of the prill. However, this approach is only partially successful and considerable amounts of dust are generated when handling the prills, especially in a windy or drafty environment.